This invention pertains to vehicle tire condition monitoring systems and more specifically to a unique sensor circuit configuration adapted to being carried on a vehicle wheel for sensing a change associated with an abnormal tire condition.
In a prior U.S. Pat. No. 3,665,387 issued May 23, 1972, to Robert S. Enabnit, there is described a signalling system for a low tire condition which incorporates a wheel sensor circuit comprising a close-circuited inductor in an inverse coil configuration for reversing the magnetic field coupling that exists between a pair of inductive coils associated with an oscillator-detector on the vehicle chassis. The close-circuited condition of the sensor circuit inductor at normal pressure periodically affects the coupling field of the oscillator coils such that a modulation pulse signal is generated and the change in condition is recognized when the modulation pulse is not present by reason of an open-circuited sensor circuit.
In a second U.S. Pat. No. 3,831,161 issued Aug. 20, 1974, to Robert S. Enabnit, an amplifier is caused to momentarily oscillate by reason of the periodic field coupling effected between input and output amplifier inductors and a close-circuited sensor circuit inductor on the vehicle wheel. The positive feedback coupling effected by the sensor circuit inductor generates a modulation pulse signal and again the change in condition is recognized when the modulation pulse is absent by reason of an open-circuited sensor circuit.
While the apparatus of these prior art patents may be adapted to sensing a change in either a pressure or a temperature condition by reason of a switch responsive to the particular condition, they do not provide for sensing changes in both pressure and temperature, either or both of which may be due to an abnormal condition occurrence.
Thus, it is an object of this invention to provide a novel sensor circuit configuration adapted for sensing the occurrence of a change in temperature associated with an abnormal tire condition.
It is another object to provide a failsafe sensor circuit configuration that is independently responsive to both pressure and temperature changes.
These objects are accomplished in a sensor circuit configuration comprising:
A. AN INDUCTIVE CIRCUIT FOR ESTABLISHING A COUPLING FIELD INDICATIVE OF A NORMAL TIRE CONDITION; AND
B. A TEMPERATURE RESPONSIVE ELEMENT THAT EXHIBITS A CHANGE-OF-STATE PHYSICAL CHARACTERISTIC IN RESPONSE TO A TEMPERATURE CHANGE ABOVE A PREDETERMINED THRESHOLD, SAID ELEMENT COUPLED INTO THE INDUCTIVE CIRCUIT TO MAINTAIN THE COUPLING FIELD INDICATIVE OF A NORMAL CONDITION WHEN ITS CHARACTERISTIC IS INDICATIVE OF A TEMPERATURE BELOW THE THRESHOLD BUT TO AFFECT THE COUPLING FIELD WHEN IT CHANGES STATE IN RESPONSE TO A TEMPERATURE ABOVE THE THRESHOLD, SAID AFFECT ON THE COUPLING FIELD BEING INDICATIVE OF AN ABNORMAL TIRE CONDITION.